The glFeedbackBuffer function
controls feedback. Feedback, like selection, is a GL mode. The mode is selected
by calling glRenderMode with GL_FEEDBACK. When the GL is in feedback mode,
no pixels are produced by rasterization. Instead, information about primitives
that would have been rasterized is fed back to the application using the
GL.

glFeedbackBuffer has three arguments: buffer is a pointer to an array
of floating-point values into which feedback information is placed. size
indicates the size of the array. type is a symbolic constant describing
the information that is fed back for each vertex. glFeedbackBuffer must
be issued before feedback mode is enabled (by calling glRenderMode with
argument GL_FEEDBACK). Setting GL_FEEDBACK without establishing the feedback
buffer, or calling glFeedbackBuffer while the GL is in feedback mode, is
an error.

When glRenderMode is called while in feedback mode, it returns
the number of entries placed in the feedback array, and resets the feedback
array pointer to the base of the feedback buffer. The returned value never
exceeds size. If the feedback data required more room than was available
in buffer, glRenderMode returns a negative value. To take the GL out of
feedback mode, call glRenderMode with a parameter value other than GL_FEEDBACK.

While in feedback mode, each primitive, bitmap, or pixel rectangle that
would be rasterized generates a block of values that are copied into the
feedback array. If doing so would cause the number of entries to exceed
the maximum, the block is partially written so as to fill the array (if
there is any room left at all), and an overflow flag is set. Each block
begins with a code indicating the primitive type, followed by values that
describe the primitive's vertices and associated data. Entries are also written
for bitmaps and pixel rectangles. Feedback occurs after polygon culling
and glPolygonMode interpretation of polygons has taken place, so polygons
that are culled are not returned in the feedback buffer. It can also occur
after polygons with more than three edges are broken up into triangles,
if the GL implementation renders polygons by performing this decomposition.

The glPassThrough command can be used to insert a marker into the feedback
buffer. See glPassThrough.

Following is the grammar for the blocks of values
written into the feedback buffer. Each primitive is indicated with a unique
identifying value followed by some number of vertices. Polygon entries include
an integer value indicating how many vertices follow. A vertex is fed back
as some number of floating-point values, as determined by type. Colors are
fed back as four values in RGBA mode and one value in color index mode.

value is a floating-point number, and n is a floating-point
integer giving the number of vertices in the polygon. GL_POINT_TOKEN, GL_LINE_TOKEN,
GL_LINE_RESET_TOKEN, GL_POLYGON_TOKEN, GL_BITMAP_TOKEN, GL_DRAW_PIXEL_TOKEN,
GL_COPY_PIXEL_TOKEN and GL_PASS_THROUGH_TOKEN are symbolic floating-point
constants. GL_LINE_RESET_TOKEN is returned whenever the line stipple pattern
is reset. The data returned as a vertex depends on the feedback type.

The
following table gives the correspondence between type and the number of
values per vertex. k is 1 in color index mode and 4 in RGBA mode.

Type

Coordinates

Color

Texture

Total
Number of Values

GL_2D

x, y

2

GL_3D

x, y, z

3

GL_3D_COLOR

x, y, z

$k$

$3 ~+~ k$

GL_3D_COLOR_TEXTURE

x,
y, z,

$k$

4

$7 ~+~ k$

GL_4D_COLOR_TEXTURE

x, y, z, w

$k$

4

$8 ~+~ k$

Feedback
vertex coordinates are in window coordinates, except w, which is in clip
coordinates. Feedback colors are lighted, if lighting is enabled. Feedback
texture coordinates are generated, if texture coordinate generation is
enabled. They are always transformed by the texture matrix.

GL_INVALID_OPERATION is generated if glFeedbackBuffer
is called while the render mode is GL_FEEDBACK, or if glRenderMode is called
with argument GL_FEEDBACK before glFeedbackBuffer is called at least once.

GL_INVALID_OPERATION is generated if glFeedbackBuffer is executed between
the execution of glBegin and the corresponding execution of glEnd.